An overlay network is a network virtualization technology that achieves multi-tenancy in cloud computing environment. Examples of overlay networks include Virtual eXtensible LAN (VXLAN), Generic Network Virtualization Encapsulation (GENEVE), and Network Virtualization using Generic Routing Encapsulation (NVGRE). For instance, VXLAN is an Open Systems Interconnection (OSI) model Layer 2 (L2) overlay scheme over a Layer 3 (L3) network. VXLAN encapsulates an Ethernet L2 frame in IP (MAC-in-UDP encapsulation) and allows Virtual Machines (VM) to be a part of virtualized L2 subnets operating in separate physical L3 networks. Similarly, NVGRE uses Generic Routing Encapsulation (GRE) to tunnel L2 packets over L3 networks.
Today, the encapsulation of an overlay network L2 frame originating from a VM is handled in software. This involves significant work on the part of the overlay network software to maintain and lookup overlay network address mapping tables, and to encapsulate a transmit packet with an outer header that includes relevant overlay network, UDP, IP and Ethernet information before it is sent out on the wire. Similarly the reverse process of decapsulation of the outer header for a received packet is also the responsibility of the software. Each mapping table lookup and encapsulation/decapsulation of packet in software incurs significant CPU cost and affects performance.
Peripheral Component Interconnect Express (PCIe) is a high-speed serial computer expansion bus standard. Single Root I/O Virtualization (SR-IOV) is an I/O virtualization technology that allows a PCIe device to appear as multiple separate physical PCIe devices.
The virtualization software (sometime referred to as a hypervisor) of some host machines support SR-IOV enabled network adapters. The instantiated VFs can be configured such that they are assigned directly to VMs and the guest operating system's driver takes possession of the VFs. While such configuration delivers near native network performance to the VM, the data path bypasses hypervisor/network stack. Hence such VMs cannot be fully utilized in an overlay network based multi-tenant environment.